170 Mr. W. S. MacLeay on the Anatomy of the 
4, The postscutellum of the mesothorax is a very remarkabie 
piece in Hymenoptera, from being in general well developed in point of 
size. It is an internal and concealed piece, running under the tergum of 
the metathorax and parallel to the medipectus. It is, I believe, an essen- 
tial character of this order that the postscutellum shall be separated from 
the scutellum except by two lateral processes. In Polistes the former 
piece is of a triangular, concave shape, the base of the triangle facing the 
scutellum and being connected with it at the angles.* Mr. Kirby does 
not appear to have noticed this important piece, as his frena in Hymen- 
optera appear to be the paraptera, so that the fraena, according to him, 
in Coleoptera (where it is the true postscutellum,) and his frena in Hy- 
menoptera, are totally distinct pieces! Our author has been led into 
this singular mistake apparently by never having dissected the thorax, 
and indeed it is rather a delicate operation to separate the metathorax 
from the mesothorax. The best mode is to make one transverse incision 
behind the scutellum and another slanting upwards under the middle pair 
of feet to meet the former, but so as not to communicate with it about 
the wings. By then breaking off the two pieces we shall have the meso- 
thorax and metathorax properly separated, that is, the upper wings with 
the mesothorax and the under with the metathorax. 
Audouin, and charges him with confounding the scutum of the mesothorax with 
the scutellum, but to what work of M. Audouin he refers Iam not aware. I 
think there must be some mistake, as the whole theory, as well as observations, 
of M. Audouin, go to separate them. M. Chabrier, Mem. du Mus., Vol. VIII. 
p. 61, says of this piece, “Ses bras semblent tendre sans cesse a s’échapper 
“en glissant des piéces entre lesquelles ils sont situés; et l’extremité de 
« chaque bras est pourvue de languettes internes qui sont tout a fait couvertes 
« par les intégumens.”’ 
* It is of the same shape in Xylocopa, and has the same kind of insertion. 
See Chabrier in Mem. du Mus. d’ Hist. Nat. Vol. VIII. tab. 4, fig. 9, where this 
piece is admirably figured, and called the costal. M. Chabrier is aware that 
it belongs to the vertebral axis, for he says, “ Je crois que ces piéces supéri- 
“ eures du trone y compris /e costal peuvent etre considerées comme des ver- 
“ ¢ébres.” The manner in which this piece articulates with the arms of the 
scutellum of the mesothorax, and with the vectiform bone of the wing ought 
to be studied in the Mémoires of MM. Juriue and Chabrier. For a figure of the 
piece in Polistes, see fig. 4, ©. See also Bennett in Zool. Journal, Vol. I.p. 397. 
